Cell cycle control is a major determinant of homeostasis during B cell development, differentiation, and tumorigenesis. Our long-term goal is to understand cell cycle control of B cell differentiation. The objective of this proposal is to elucidate the mechanism by which cyclin-dependent kinase (CDK) inhibitors control cell cycle progression and cellular differentiation in the generation of plasma cells. In humoral immunity, the generation of a primary antibody response requires activation and expansion of antigen-specific B cells, followed by terminal differentiation to antibody secreting plasma cells. In successive immunizations, the antibody response is accelerated and amplified by the activation of long-lived, antigen-specific memory B cells. Plasma cells are arrested in the G1-phase of the cell cycle, as are memory cells. Although timely cell cycle entry and exit are undoubtedly crucial for this dynamic process, the precise mechanism is not known. In preliminary studies, we have demonstrated that cell cycle arrest mediated by a specific CDK inhibitor, p 18 INK4c, is required for the final differentiation of non-secreting plasmacytoid cells to antibody-secreting plasma cells in the T-dependent humoral immune response. This requirement is differentiation stage-specific, because p 18 INK4c is dispensable in Ig switch recombination and variable region hypermutation, in reactivation of memory cells, and in differentiation of antigen-specific, switched B cells to plasmacytoid cells. Our results provide the first direct evidence for cell cycle control of B cell immunity. They suggest that cellular differentiation to functional plasma cells is contingent upon cell cycle arrest mediated by specific CDK inhibitors. The requirement for p 18 INK4c at a specific B terminal differentiation stage, the availability of CDK inhibitor-deficient mice and of an in vitro B cell terminal system present a unique opportunity to test this hypothesis. We propose to determine 1) the cell type and signaling pathway specificity with which CDK inhibitor p 18 INK4c controls cell cycle progression and the generation of Ig-secreting plasma cells; 2) cooperative cell cycle control of plasma cell generation by CDK inhibitors p18 INK4c and p27Kip1; 3) the control of differentiation of plasmacytoid cells to Ig secreting plasma cells by p18INK4c and IL-6 signaling. Since no information is available regarding the roles of CDK and CDK inhibitors in an immune response, successful completion of this proposal will represent a significant advance in understanding the fundamental mechanisms in B cell terminal differentiation. It will also provide a framework for future studies of cell cycle control in B cell immunity, the formation of memory B cells in particular, as well as cell cycle dysregulation in B cell and plasma cell tumorigenesis.